1. Field of the Invention
The present invention relates to a transfer method using a transfer roller that can be applied to electrophotograhic apparatuses such as copiers, printers, facsimiles, etc.
2. Description of the Prior Art
In an electrophotographic apparatus as represented by a copier, the surface of a photosensitive material drum is electrically charged uniformly, an electrostatic latent image corresponding to the original image is formed by light-exposure on the surface of the photosensitive material drum, the electrostatic latent image is developed to form a toner image on the surface of the photosensitive material drum, the toner image is transferred from the surface of the photosensitive material drum onto a transfer paper, and the transferred toner image is fixed by the application of heat and pressure to form a copy image.
In such an electrophotographic apparatus, a corona charger (transfer charger) or a transfer roller is widely used as a means for transferring the toner image formed on the surface of the photosensitive material drum onto the transfer member (paper).
According to the method which uses the corona charger, the back surface of the transfer member is electrically charged by corona discharge to a polarity opposite to that of the toner image, and the toner image is transferred by this electric charge onto the surface of the transfer paper.
According to the method which uses the transfer roller, a voltage is applied to the roller so that the roller is maintained at a potential of a polarity opposite to that of the toner image, the transfer paper is passed in this state between the photosensitive material and the transfer roller, and the toner image is transferred onto the surface of the transfer paper by an electric field established between the transfer roller and the toner image.
According to the transfer method which uses the corona charger, however, the transfer paper is electrostatically adsorbed by the surface of the photosensitive material drum. In order to separate the transfer member on which the toner image is transferred from the surface of the photosensitive material drum, therefore, an AC charger must be employed causing the transfer mechanism to become complex. There further arouses a problem of generation of ozone due to corona charging.
According to the transfer method which uses the transfer roller, on the other hand, the transfer paper can be separated from the surface of the photosensitive material drum relatively easily without the need of using the corona charger such as AC charger as a separation mechanism and without generating ozone, giving advantage over the method that uses the corona charger.
When the toner image is to be transferred onto the transfer paper, however, paper powder (paper dust) generating from the transfer paper brings about a problem. That is, when the paper powder adheres on the surface of the photosensitive material drum, it has been known that the image becomes defective due to defective cleaning and a drop in the main charging potential on the surface of the photosensitive material drum. In particular, when the toner image is transferred by using the transfer roller, the paper powder is formed conspicuously.
Recently, furthermore, it is a tendency to employ a toner recycling method for recycling the residual toner recovered by the cleaning device into the developing device to reuse it. When this method is employed, however, the generation of paper dust becomes a problem.
It therefore becomes necessary to remove the paper powder from the transfer paper prior to transferring the toner image.
The most generally employed method of removing the paper powder consists of passing the transfer paper between a pair of insulating rollers to scratch the paper powder off the transfer paper by the rollers while imparting a frictional charge thereto.
According to the above method that uses the pair of insulating rollers, however, it is not allowed to remove the paper powder from the transfer paper to a sufficient degree, and an improvement has been desired. In particular, this tendency becomes conspicuous when the developing is effected by using a toner that is positively charged.